1. Field of the Invention
This invention pertains to fiber optic cable constructions, and particularly to a dry, water blocking loose tube cable suitable for use at low temperatures.
2. Discussion of the Known Art
The infiltration of water inside a fiber optic cable can degrade the performance of the cable significantly, especially in environments where temperatures are at or below freezing. Passage of water internally over the length of the cable may also have adverse effects on cable end connectors and/or associated terminal equipment.
In the presence of moisture, glass fibers contained inside a fiber optic cable will tend to fracture even when stressed below their theoretical maximum tensile limit, due to surface flaws whose effects are aggravated by the moisture. Long lengths of glass fibers in the cable may therefore break after undergoing a strain of only about 0.5 percent elongation.
In view of the above, fiber optic cables have been constructed in ways that reduce or block the passage of water inside the cable, and minimize any stress transferred to internal fibers during cable installation and use. See, e.g., U.S. Pat. No. 4,909,592 (Mar. 20, 1990) and U.S. Pat. No. 5,630,003 (May 13, 1997), which are incorporated by reference. Both patents disclose cable arrangements in which tapes or yarns containing or coated with a super absorbent composition, are disposed in proximity to optical fibers inside the cable. The tape or yarn swells upon contact with water, and, thus, physically reduces or blocks continued passage of the water interiorly of the cable. Other water blocking means include the use of petroleum based or jelly-like filling materials, sometimes called “filling” compounds. These materials must be removed with a cleaning agent or solvent when a cable is opened at one end to carry out a splicing operation, however. The use of such solvents has a drawback in that color dyes or other indicia provided on the individual fibers for identification may also be removed. Accordingly, an all dry, water blocking cable configuration that allows splicing without requiring special cleaning agents to dissolve greasy filling compounds inside the cable, is most desirable.
The mentioned U.S. Pat. No. 5,630,003 is directed to a so-called loose tube fiber optic cable, wherein one or more flexible buffer tubes inside the cable each contain a set of optical fibers that extend through a passage of each tube. In a loose tube fiber optic cable, a strong but flexible central rod acts to resist tensile loads applied to the cable, i.e., the rod acts as a strength member. At the same time, the rod serves as an organizer about which the individual fiber-containing buffer tubes are helically wound. The rod is formed typically of, e.g., glass reinforced polyester (GRP) and is sheathed with a layer of polyethylene (PE). The use of glass for the central rod prevents the cable from behaving as an electrical conductor, and, therefore, minimizes the susceptibility of the cable to lightening or other unintentional contact with a potentially damaging electrical source.
In the cable of the '003 U.S. patent, a number of strands or filaments of an absorbent fiber yarn are disposed in the passage of each buffer tube. Also, dry strips impregnated with a water absorbent composition (e.g., DryBlock® available from OFS Fitel USA) are arranged to fill voids that would otherwise form inside the cable, outside of the buffer tube walls. A cable similar to that disclosed in the '003 U.S. patent is available from OFS under the trademark “Fortex DT”.
The dry loose tube fiber optic cable configuration described above requires that a water absorbent member (e.g., a coated yarn) be disposed in the passage of each buffer tube during cable production. Therefore, the member must not only be physically compatible with the buffer tube, but should not affect the transmission characteristics of the optical fibers inside the tube significantly either before or after the member swells to block water movement through the tube passage.
Dry, water absorbent materials used in fiber optic cables are typically particulate in nature. If such particles are coated on a yarn, and the coated yarn is then placed in the passage of a buffer tube, the particles will likely come into direct contact with the fibers in the same passage. It has been found that such contact induces macrobending loses at very low temperatures, resulting in unacceptable increases in fiber attenuation.
Fiber optic cables are manufactured to meet various industry standards one of which is Telcordia® GR 20, Issue 2 (1998)(“the Telcordia standard”). This standard, which is incorporated by reference, requires a cable to maintain prescribed performance parameters over a temperature range of from −40 to +70 degrees C. As far as is known, a dry, loose tube fiber optic cable that can maintain the prescribed parameters with respect to water penetration, fiber attenuation, and tensile strength, when cycled to a temperature as low as −60 degrees C., has not been commercially produced. Nevertheless, there is a significant market for such a cable in regions where fiber optic cables are deployed at temperatures as low as −60 degrees C.